Two methods are chiefly employed today to prepare ingots from molten scrap of refractory metals such as titanium, zirconium and alloys thereof. In one method, scrap in the form of cuttings or small lumps is supplied into the hearth of a vacuum melting furnace and is caused to melt by application of a electron beam or plasma electron beam, and the molten puddle in the hearth is transferred into a water-cooled mold and the solidified casting withdrawn from the bottom of the mold. The second method employs arc melting with a consumable electrode formed of the metal scrap which is suspended in a water-cooled copper crucible. A d.c. arc is produced between the tip of the electrode and the seed melt in the crucible, and as the electrode is gradually melted, it drops into the crucible and builds up on its bottom. The molten pool is solidified from the bottom upward into an ingot.
These methods, however, have various problems. In the first method using an electron beam or plasma electron beam in a vacuum melting furnace the scrap must be graded according to size. In addition, this method is capable of handling only scrap of small size ranging from cuttings to small lumps and much time is required to obtain scrap which is uniform in size. In short, the first method is disadvantageous in that it is capable of handling only a limited range of scrap. In the second method which depends on arc melting using a consumable electrode, the process of electrode fabrication must be changed according to the size of scrap and this requires the scrap to be graded. Scrap in the form of cuttings or small lumps requires the additional step of pressing, and lumps of a medium size must be cut to smaller sizes prior to pressing. Therefore, a lot of steps and time are involved in the fabrication of a consumable electrode.
The present inventors therefore made concerted efforts to solve the aforementioned problems and found that such problems can be eliminated by one of the following methods. In one method, unsorted metal scrap of various sizes is charged into a tubular member with a closed end which is made of the same material as that of the metal scrap, and this tubular member while being held horizontally in a vacuum melting furnace is melted with a electron beam or plasma electron beam. In the other method, two tubular members filled with the metal scrap are used as a pair of consumable electrodes which are positioned in a face-to-face relationship in such a manner that an arc is produced between the tips of the electrodes to melt the metal scrap in the tubes.
The present inventors continued their studies and obtained the following additional observations:
(1) If, instead of the tubular member, use is made of a box with its top open which has a shape such a that formed by cutting a tube of a polygonal cross section in the axial direction, metal scrap can be easily charged into the box from the top. The state in which scrap is packed in the box can be visually inspected and the scrap can be charged at will into an part of the box such that uniform and dense packing is attained with great ease. Additional advantages of using the box with its top open are that the box can be fabricated with less material than is required in making a tube and that considerable reduction in the manufacturing cost is realized.
(2) If the material of the box is not the same as that of the metal scrap to be charged thereinto, the impurities will change the final composition of the ingot formed from a molten mixture of the scrap and the box. However, if the resulting variation in the composition of the ingot is within tolerable limits, the box does not need to be formed of a material which is completely identical to that of the metal scrap, provided that said box material contains at least one of the components which are present in the metal scrap.